Ventilating apparatus having a closed breathing circuit are preferably utilized in anesthesia treatment because this kind of apparatus configuration assures that the following can be realized: a smallest possible consumption of fresh gas (for example, respirating gas enriched with oxygen); or a smallest possible consumption of anesthetic gases and anesthetic agents; and, maintaining respirating gas moisture and respirating gas temperature during an anesthesia. Carbon dioxide is removed from the respirating gas exhaled by the patient and the consumption of, for example, oxygen and possibly anesthetic gas is compensated by metering fresh gas. For this purpose, it is necessary only to meter so much fresh gas as corresponds to the actual consumption and leakage which possibly occurs.
In contrast and for ventilating apparatus having a semi-closed breathing circuit, only a portion of the breathing gas exhaled by the patient is again returned. For this purpose, the fresh gas flow is adjusted substantially higher than the actual consumption which effects a thinning of the exhaled respirating gas. The excess consists of a mixture of fresh gas and exhaled respirating gas. This excess is vented via a valve after each breath. The excess gas therefore is lost and must be eliminated by means of complex removal devices such as a suction device. The venting of excess gas also results in an excessive consumption of anesthetic gases and anesthetic agents.
A closed circuit anesthesia apparatus is disclosed in European Patent 0 121 255 and uses a complex control loop for metering fresh respirating gas. The respirating gas components are replenished in the control loop with corresponding sensors which are used for driving the metering unit. In this way, the level of the loop conducting the respirating gas is determined at every instant during a ventilation and the required quantity of fresh gas together with the anesthetic gas is supplied.
A metering of this kind is however complex and expensive since a measuring sensor is needed for each gaseous component present in the composition making up the respirating gas so that after a completed measurement, the corresponding quantity of respirating gas can again be resupplied. In a closed breathing circuit, only several 100 ml of respirating gas are used for each breath; but the entire respirating gas system has a substantially greater content with respect to its volume. For this reason, for a precise metering of such small quantities it is required that the compliance changes as little as possible in the course of a ventilation. An increased compliance would have as a consequence that only a small metered quantity would correspond to that which was actually consumed and the excess would actually increase the pressure in the breathing system.
A compliance change of this kind is facilitated in the known ventilating system in that a reservoir having a changing volume is continuously connected to the breathing gas line during the entire breathing cycle whereby its content changes in accordance with consumption. A further volume change occurs because the elastic components such as the breathing hoses in the breathing loop are increased with respect to their volume because of the ventilating pressure. This effect is especially noticeable during the inspiration phase. The metering of the consumed respirating gases thus takes place in a breathing circuit of variable compliance.